# [Help Physics] Torsion pendulum

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#### baTTy

##### Newbie level 3

(sorry about the 'drawing' was something I put together on paint just fast to give an idea of the pendulum I'm talking about because I came across several designs. Basically it's a stretched wired stuck between two plates, with a handle bar in the middle which you use to rotate the wire. The bar has several slots for masses so you can vary the distance of the masses to the central axis and the weigth you put on each side)

So, if I have:
the length of the wire - L
the diameter of the wire - D
the weight of each mass (ring) - M
the distance of the each ring to the center - C
and the moment of inertia with the rings - In

how can I calculate the period (T), the torsion constant of the wire (K) and the initial moment of inertia (that is the moment of inertia of the wire and handle bar without rings - I0)?

##### Full Member level 2
You will need to know the spring properties of the metal in the wire. I would look up Young's modulus and then go for my copy of Roark's formulas for stress and strain (ISBN-13: 978-0071742474). You might find some help here: Roark's Formulas for Stress and Strain

#### baTTy

##### Newbie level 3
ok, thank you, ill give it a look

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